Soluble molecular catalysts, particularly complexes of transition metals are well known in the art. Such catalysts are also known to catalyze a variety of useful organic transmformations. These transformations for instance include hydrogenation, hydroformylation, carbonylation, amination, isomerization, telomerization, Heck olefination, Suzuki coupling, metathesis, epoxidation etc. Such transformations find a variety of useful applications for the synthesis of pharmaceuticals, pesticides, solvents and other valuable products of industrial and consumer significance.
Amongst the established practices known in the prior art, catalytically active transition metal complexes have principally been applied in homogeneous form, as solution in a reactant phase. For example, in case of hydroformylation of olefins using rhodium and phosphine ligand complex catalyst wherein phosphine ligand is free of ionic charge such as tributyl phosphine, triphenyl phosphine etc. and soluble in the reaction medium. Although such catalysts are highly effective, in terms of productivity and selectivity, its applicability on practical grounds is often limited to volatile products. In case of reactions catalyzed by homogeneous catalysts involving high molecular weight and especially nonvolatile products catalyst separation is a critical problem. High cost of catalyst, susceptibility to high temperatures and stringent product specification demand quantitative catalyst separation. Common unit operations such as distillation and crystallization are least significant since, organometallic complexes being delicate in nature and cannot withstand separation stresses especially thermal stresses as encountered in distillation. Other separation techniques being inefficient in separating such a small quantity of catalyst cannot be used in effective manner. Moreover high purity of the product is of importance in products such as pharmaceuticals, demanding rigorous separation of catalyst from product stream. Thus use of homogeneous catalyst as such has suffered from inherent difficulties in the recovery of the catalysts from reaction products. Efficient catalyst recovery and recycle is the pivotal issue for the economic viability of the process since, complexes and ligands are often expensive.
It is also known in the art to use aqueous solutions of sulfonated aryl phosphines and many other water-soluble compounds and transition metal complex catalyst derived from it to effect reactions. As disclosed in patent (U.S. Pat. No. 4,248,802) all such reactions are operated in biphasic conditions wherein catalyst phase is aqueous and products and reactants dissolved in organic phase. Similarly reverse biphasic techniques are also applicable wherein catalyst is dissolved in organic phase and product and reactants in aqueous phase. A judicious choice is necessary while utilizing biphasic catalytic systems depending upon solubility of reactants and products. In either case at the end of reaction catalyst and product phases are separated wherefore catalyst phase is recycled and product phase is directed for further downstream processing.
It is however recognized that catalytic activity is low in biphasic medium due to limited solubility of organic reactants in the catalyst phase. Moreover such biphasic reactions require high reactor pressure in case of gas-liquid reactions. To achieve practical rates of reactions catlayst loading has to be increased or alternatively using larger process equipment, which is usually cost prohibitive. Further, these reactions require numerous accessory devices to separate liquid-liquid fractions under reaction conditions.
Over the past quarter of century many attempts have been made to heterogenize this versatile class of soluble catalysts. Several methods were developed with central theme being retention of high activity and selectivity as that of native catalytic species and facilitate separation by simple filtration, centrifugation or gravity settling.
One of the techniques to form a solid catalyst involves interaction of metal salt or precursor complex with solid support that is appropriately functionalized with organic functional groups that are capable of forming coordination bonds with metal. The support used in this context is either organic-polymeric or inorganic matrix. These supports are chemically functionalized to bear amino, phosphino and carboxylato functional groups on the surface of the support. Work related to this technique is described in Catalysis Reviews, 16, 17-37 (1974); Chemical Reviews, 81, 109, (1981); Tetrahedron Asymmetry, 6, 1109-1116 (1995); Tetrahedron Letters, 37, 3375-3378 (1996). “Catalysis by supported complexes”, Studies in surface science and catalysis, volume 8, Elsevier Publishing Co. Amsterdam, 1981 describes the complexes grafted to inorganic supports.
From practical stand point these catalysts are not widely used since their activities are frequently lower than corresponding homogeneous catalysts in addition there are various complications that are inherent due to polymeric nature of the support for example swelling and shrinking of the matrix, which alters diffusion resistance. It is also found that in long run and upon exposure to oxygen metal attached to support is lost in the solution thereby degeneration of the activity of the catalyst.
Supported liquid phase catalyst such as those described in U.S. Pat. No. 3,855,307 (1974) and U.S. Pat. No. 4,994,427 (1991) are critically sensitive to the character of the reaction medium and are often leached in to reaction medium depending upon the nature of the solvent. The applicability of such catalyst is limited to only vapor phase reactions. The technique as described in U.S. Pat. No. 4,994,427(1991) wherein solution of water-soluble catalyst is distributed on high surface area solid. The aqueous film of catalyst containing solution remains insoluble in nonpolar organic phase thus, after reaction solid catalyst can be recovered by simple filtration. Appllicability of such catalyst is limited to reactions involving water insoluble reaction media. Moreover such catalysts are sensitive to content of water.
Entrapment of the catalyst in porous material such as zeolite has been described by Balkus, et al in J. Inclusion Phenom. Mol. Recognit. Chem., 21(1-4), 159-84 (English) 1995 The catalyst is encapsulated in three-dimensional network of zeolites wherein, catalyst because of size exclusion can not diffuse out of zeolite but smaller sized reactants diffuse inside the zeolite and products formed subsequently diffuse out. Yet another article J. Catal, 163(2), 457-464 1996 have described the method to entrap catalyst within the polymer matrix but because of diffusion resistance, catalyst efficiency is doubtful.
Despite several known techniques for heterogenization of soluble molecular catalysts there is no known method, which can be conveniently used for diversity of catalytic entities using a common protocol. Furthermore catalyst formed by such protocol is required to provide a solid catalyst that can be used for polar as well as nonpolar reaction media. Certainly a particular need exists for such technique of catalyst formulation and present invention is aimed to fulfill these needs.